MERV Ratings: Where Bigger Doesn’t Always Mean Better

Minimum Efficiency Reporting Value (MERV) ratings offer an easy-to-read roadmap for choosing the air filter that fits your home's filtration needs. But choosing your home's next air filter isn't as simple as picking the filter with highest number.

It's also important to understand the impact that MERV ratings can have on your HVAC system and how choosing a filter with a too-high rating for your system can cause long-term problems that affect its performance and your home's indoor air quality.

MERV Ratings At A Glance

MERV was conceived by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) as a simple and effective way for homeowners to compare air filter performance among different types and brands. Unlike other rating systems, the MERV rating scale measures the worst possible performance of a filter when it comes to removing airborne particles (some as small as 0.3 microns in size). These particles include dust, pollen, pet dander, tobacco smoke and even bacteria and viruses.

With MERV, air filters are rated on a scale of 1 to 16. As MERV ratings increase, fewer airborne contaminants are able to successfully pass through the filtration media. Higher MERV ratings are a good thing, in most respects, but automatically choosing the highest-rated MERV filter has quite a few unexpected drawbacks.

Why The Highest Number Isn't Always The Best

The farther you go up the MERV ladder, the more effective your filtration options become, right? As it turns out, it's not as straightforward as you'd think.

Pore size has plenty to do with the filter's ability to block and trap airborne particles. Smaller pores trap more particles, but smaller pores also create greater air flow resistance. Too much air flow resistance creates damaging pressure differentials that add unnecessary wear and tear to your blower fan motor while reducing your home's air quality.

Filters with high MERV ratings are more likely to feature smaller pores which, in turn, constrict air flow in your HVAC system. For instance, an air filter rated at MERV 15 offers less overall air flow than one rated at MERV 8. If your HVAC system was designed with air filters at MERV 8 and below, long-term use of a MERV 15 filter can actually shorten the system's lifespan by a significant degree.

According to many experts, going beyond MERV 13 for residential filtration offers vanishingly small returns for improving indoor air quality. Filters rated between MERV 7 and 13 offer just as effective filtration as a true HEPA filter while allowing for better air flow and quieter operation.  

Choosing The Right Air Filter For Your HVAC System

Air flow is crucial to your HVAC system's operation. For this reason, you should select an air filter that offers excellent filtration without compromising on air flow. For instance, a conventional pleated air filter rated at MERV 8 offers improved performance over a typical fiberglass filter without sacrificing air flow.

Here are a few suggestions to follow when choosing the right filter for your system:

  • Find out what your manufacturer recommends. In most cases, you can find the MERV requirements for your HVAC system in your owner's manual or service booklet.
  • See what your technician recommends. Your seasoned HVAC technician can offer plenty of in-depth advice on MERV ratings and help you choose the air filter with compatible performance for your HVAC system.
  • Steer clear of true HEPA filters unless you have a HVAC system that's designed for it. Using a true HEPA filter on a conventional HVAC system causes air restrictions that degrade your system's performance and wastes energy.

Choosing the best available filter is a balancing act between proper air flow and filtration performance. Going for the highest rating isn't always the best choice. Instead, an air filter with a mid-range MERV rating provides the effective filtration your system needs without putting the squeeze on your HVAC system. For additional info, contact a local HVAC specialist.